1. Field of the Invention
The present invention relates to a thermal head and a printer including the same.
2. Description of the Related Art
There has been conventionally known a thermal head for use in thermal printers, which performs printing on a heat-sensitive recording medium such as paper by selectively driving some of a plurality of heating elements based on printing data (see, for example, Japanese Patent Application Laid-open No. 2007-83532).
In the thermal head disclosed in Japanese Patent Application Laid-open No. 2009-119850, a thin glass plate is bonded to a substrate in which a concave portion is formed, and heating resistors are provided on the thin glass plate, whereby a cavity portion is formed in a region of the substrate, which corresponds to the heating resistors. This thermal head allows the cavity portion to function as a heat-insulating layer having a low thermal conductivity, and reduces an amount of heat flowing from the heating resistors to the substrate, thereby improving thermal efficiency and reducing power consumption.
For example, as disclosed in Japanese Patent Application Laid-open No. Hei 06-298539, for bonding pieces of glass to each other, substrates subjected to mirror polishing are used in order to obtain smooth substrate surfaces. It is difficult to manufacture a thin glass plate having a thickness of 100 μm or less, and it is difficult to handle the thin glass plate in a manufacturing process of the thermal head. Therefore, a material glass plate having a thickness allowing relatively easy handling thereof is bonded to the substrate, and thereafter, is processed to a desired thickness by mechanical polishing or the like, whereby a thin glass plate having the thickness of 100 μm or less is realized.
Incidentally, in the mechanical polishing, in order to form a glass substrate, which is obtained by bonding the material glass plate and the substrate to each other, to a desired thickness, a two-stage-process polishing operation is performed, in which second-stage finish polishing is performed after first-stage rough polishing. In this case, the finish polishing or the like is performed for a surface of the substrate, the surface roughness of which is increased by the first-stage rough polishing, and the surface of the glass substrate is finished into mirror surface.
However, the glass substrate the thickness of which is reduced by the first-stage rough polishing are decreased in strength, and accordingly, an apprehension that the glass substrate may be broken at the time of the subsequent finish polishing is increased. Further, in the finish polishing, polish grain is fine, and accordingly, it is necessary to increase load applied to the substrate as compared with the case of the rough polishing. Therefore, at the time of the finish polishing, a large tensile stress occurs in a portion of the thin glass plate, which faces to the cavity portion. In particular, many cracks are included in a surface of the thin glass plate processed by the mechanical polishing or the like, there is a problem in that the thin glass plate is prone to break when the cracks grow.
Further, a printer that mounts the above-mentioned thermal head thereon has a structure in which thermal paper is pressed against a platen roller in a sandwiched manner. Hence, the heating resistors of the thermal head are pressed against the thermal paper with predetermined pressing force by a pressure mechanism. In particular, in the case where minute foreign matters each having a size ranging from several micrometers to several ten micrometers are interposed between the platen roller and heater portions, an extremely large tensile stress occurs in the portion of the thin glass plate, which faces to the cavity portion. Thus, the thin glass plate is prone to be broken.
Meanwhile, in order to prevent such a breakage of the thin glass plate, it is necessary to ensure the strength of the thin glass plate. However, in accordance with the conventional thermal head, the thin glass plate must be thickened in order to ensure the strength of the thin glass plate, and accordingly, there is a disadvantage of decreasing thermal efficiency of the thermal head because an amount of heat transfer from the heating resistors is increased.
The present invention has been made in view of the above-mentioned circumstances. It is an object of the present invention to provide a thermal head that has a cavity portion at a position corresponding to heating resistors and is capable of improving thermal efficiency while ensuring strength of the cavity portion, and a printer including the thermal head.